Automotive vehicle engines are generally equipped with fuel injectors for injecting fuel (e.g., gasoline or diesel fuel) into the individual cylinders or intake manifold of the engine. The engine fuel injectors are coupled to a fuel rail which contains high pressure fuel that is delivered by way of a fuel delivery system. In diesel engines, conventional fuel injectors typically employ a valve needle that is actuated to open and to close in order to control the amount of fluid fuel metered from the fuel rail and injected into the corresponding engine cylinder or intake manifold.
One type of fuel injector that offers precise metering of fuel is the piezoelectric fuel injector. Piezoelectric fuel injectors employ piezoelectric actuators made of a stack of piezoelectric elements arranged mechanically in series for opening and for closing an injection valve needle to meter fuel injected into the engine. Piezoelectric fuel injectors are well known for use in automotive engines.
The metering of fuel with a piezoelectric fuel injector is generally achieved by controlling the electrical voltage potential applied to the piezoelectric elements to vary the amount of expansion and contraction of the piezoelectric elements. The amount of expansion and contraction of the piezoelectric elements varies the travel distance of a valve needle and, thus, the amount of fuel that is passed through the fuel injector. Piezoelectric fuel injectors offer the ability to meter precisely a small amount of fuel.
Typically, the fuel injectors are grouped together in banks of one or more injectors. As described in EP1400676, each bank of injectors has its own drive circuit for controlling operation of the injectors. The circuitry includes a power supply, such as a transformer, which steps-up the voltage VS generated by a power source, i.e. from 12 Volts to a higher voltage, and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors which are used to power the charging and discharging of the piezoelectric fuel injectors for each injection event. Drive circuits have also been developed, as described in WO 2005/028836A1, which do not require a dedicated power supply, such as a transformer.
The use of these drive circuits enables the voltage applied across the storage capacitors, and thus the piezoelectric fuel injectors, to be controlled dynamically. This is achieved by using two storage capacitors which are alternately connected to an injector arrangement. One of the storage capacitors is connected to the injector arrangement during a discharge phase when a discharge current flows through the injector arrangement, initiating an injection event. The other storage capacitor is connected to the injector arrangement during a charging phase, terminating the injection event. A regeneration switch is used at the end of the charging phase, before a later discharge phase, to replenish the storage capacitors.
Like any circuit, faults may occur in a drive circuit. In safety critical systems, such as diesel engine fuel injection systems, a fault in the drive circuit may lead to a failure of the injection system, which could consequentially result in a catastrophic failure of the engine. A robust diagnostic system is therefore required to detect critical failure modes of piezoelectric actuators, and of the associated drive circuits, particularly whilst the drive circuit is in use.
An aim of the invention is therefore to provide a diagnostic tool that is capable of detecting critical failure modes, or fault response characteristics, of an injector arrangement, and the associated drive circuit, and a method of operating the diagnostic tool.